In my view it is simple. When an elder is made aware of a crime, he should tell the person that he is not authorized in this matter and that clergy confidentiality does not apply here. And then stop the discussion, tell the person to go to the police and if the person does not, tell the person that you are going to report him.
It is like with hospitals and doctors. There is a medical confidentiality unless crime is involved (shot wound, child molestation or home violence etc). In such cases, hospital or doctor is obliged to report.
Even if the law would not really oblige a clergy to report what is considered a crime, it should be his Christian conscience. But it all start with the clergy to say that the conversation is not subject to clergy confidentiality if the person is going to tell of a crime he committed.
Unfortunately, many elders (WBTS and other religions) feel they are above the secular laws and even above the law of Christ and act as they are capable and serve as police officer, detective, judge, jury and...oh yes, clergy.